1. Technical Field of the Invention
The present invention relates generally to a sensor element of a gas sensor which may be employed in air-fuel ratio control of internal combustion engines.
2. Background Art
A typical gas sensor to be disposed in an exhaust system of an internal combustion engine for controlling the air-fuel ratio has a sensor element which consists essentially of a solid electrolyte body, a target gas electrode, and a reference gas electrode.
The target gas electrode is disposed within a gas chamber filled with a gas to be measured and generally has a protective layer formed on an outer surface thereof. The protective layer is usually made of a porous ceramic material.
The solid electrolyte body is usually made of an oxygen ion conductive material such as a ceramic material which is produced by forming and baking ZrO2 powder.
The target gas electrode and the reference gas electrode are exposed to the gas to be measured and a reference gas such as air, respectively, and produce an output as a function of the concentration of a specific component contained in the gas such as oxygen. The measurement of the specific component may be achieved by applying the voltage to each of the target gas electrode and the reference gas electrode or in another known manner.
The target gas and reference gas electrodes are each designed to perform a catalyst function to facilitate average reaction of the gas to be measured, which enables, for example, the concentration of oxygen contained in exhaust gasses of the internal combustion engine to be measured with high accuracy for determination of the stoichiometric.
Typically, each of the target gas electrode and the reference gas electrode is made in (1) a chemically plating manner or (2) a baking manner in which a paste containing an electrode material is applied to a solid electrolyte body and baked.
In the former manner (1), in order to avoid aggravation or cohesion caused by exposure to the heat during use of the gas sensor, after plated, the electrode needs to be subjected to heat treatment at a temperature higher than that to which the gas sensor is exposed actually.
In the latter manner (2), the solid electrolyte body to which the paste is applied is baked usually at temperatures of 1300 to 1500xc2x0 C.
It is known in the art that the above heat treatment and baking result in inactivation of the electrode, which will lead to a decrease in response of the sensor element. In order to avoid this drawback, and to improve electrode response, it has been proposed to expose the electrode to a strong reducing atmosphere with H2 or CO treatment to highly activate the electrode.
In recent years, however, the emission regulations have been made more rigorous, and burn control gas sensors installed in an exhaust system of an internal combustion engine are required to have their response increased more than before.
The improvement of the response by the above reducing treatment has its limit. Another method of increasing the response of the sensor element is, therefore, sought.
The improvement of the response rate of the gas sensor may also be achieved by forming a large number of pores in a protective layer provided on the electrode to improve the degree of diffusion of the gas. The protective layer, however, must be designed to protect the electrode against noxious compositions contained in the gas to be measured as well as avoidance of thermal cohesion. Usually, exhaust gasses within an exhaust pipe of an internal combustion engine contain much poison such as Pb compounds produced from gasoline and Si compounds generated from the exhaust pipe. The protective layer will facilitate ease of penetration of the poison through the protective layer to the electrode, thus resulting in premature deterioration of the electrode.
It is therefore a principal object of the invention to avoid the disadvantages of the prior art.
It is another object of the invention to provide a gas sensor element which exhibits a rapid response and a higher heat resistance.
According to one aspect of the invention, there is provided a gas sensor element which comprises: (a) a solid electrolyte body; (b) a target gas electrode provided on a surface of the solid electrolyte body so as to be exposed to a gas to be measured; and (c) a reference gas electrode provided on a surface of the solid electrolyte body so as to be exposed to a reference gas. Each of the target gas electrode and the reference gas electrode is made up of a plurality of crystal grains defined by grain boundaries. The total length of the grain boundaries in each of the target gas electrode and the reference gas electrode is 1000 xcexcm or more in a surface area of 1000 xcexcm2.
In the preferred mode of the invention, the number of pores formed in the target gas electrode whose diameters are within a range of 0.1 xcexcm to twice the thickness of the target gas electrode are 5 to 100 in a surface area of 1000 xcexcm2.
The number of pores formed in the reference gas electrode whose diameters are within a range of 0.1 xcexcm to twice the thickness of the reference gas electrode are 5 to 100 in a surface area of 1000 xcexcm2.
Some of the pores exist in grain boundaries of each of the target gas electrode and the reference gas electrode.
According to another aspect of the invention, there is provided a method of producing a gas sensor element which includes a solid electrolyte body, a target gas electrode provided on a surface of the solid electrolyte body so as to be exposed to a gas to be measured, and a reference gas electrode provided on a surface of the solid electrolyte body so as to be exposed to a reference gas. The method comprises the steps of exposing the gas element to a gas atmosphere containing at least one of a hydrocarbon gas, a CO gas, and a H2 gas at 400 to 900xc2x0 C. and applying an ac voltage to the target gas electrode and the reference gas electrode. This causes pores to be formed in the target gas electrode and the reference gas electrode.